Surface acoustic wave (SAW) and micro-electromechanical (MEMS) devices, by way of example, are in a subgroup of electronic devices where the active area must move freely for proper functioning of the device. By way of example, a surface acoustic wave (SAW) resonator typically includes transducers and reflectors disposed upon a piezoelectric substrate. The transducer is made up of interdigital electrodes of metal such as aluminium, copper, magnesium or metal alloy. Lithium tantalate, lithium niobate and quartz are commonly used piezoelectric substrates for SAW devices. When an RF electric field is applied across the input transducer, acoustic waves are generated and travel along a top surface of the piezoelectric substrate. These waves are detected and processed by the interdigital electrodes to provide a filtering device. A space above an active region of the SAW device is needed to avoid dampening the propagation of the surface acoustic waves. To provide environmental protection, these devices are hermetically sealed into a cavity of a ceramic package. Electrical connections to the SAW devices may be made through interconnects embedded in the ceramic package. Such an approach can result in stacked measurements or tolerances that include the thickness of the outer package, the gap between a device wafer and the bottom of the package, the thickness of the device wafer, the gap between the device wafer and the lid, and the lid itself. There is a technological desire to reduce the overall package height and maintain low cost. One consideration is to eliminate the ceramic package.
Integrated circuit packaging typically achieves minimal package height by using an epoxy that covers the device to provide environmental and mechanical protection. This approach is not practical for the SAW and MEMS devices as the epoxy material would completely cover the device and impede proper functioning. However, wafer bonding technologies have been widely used in the MEMS arena, and to a limited extent in the SAW device field, to create a cavity around the active area. Given that most bonding techniques take place at temperatures above room temperature, the resulting effects of a coefficient of thermal expansion (CTE) of the device material and the cap material must be matched or mitigated. Matching the CTE typically means using the same material for both the cap and the device.
Since the cap material does not require the same acoustic properties as the substrate, a lower cost material may be used if the material bonding technique and preparation can accommodate the thermal expansion mismatch. This approach is not practical for certain devices of interest as the epoxy material would completely cover the device and impede the desired function. An effect of bonding the cap material to the device material with different thermal coefficient of expansions results in a misalignment of wafers. By way of example, while alignment marks for the cap material and the device wafer are aligned at room temperature, at an elevated bonding temperature, due to the different physical expansion of the two substrates, a misalignment and off-registry of the alignment results.